Messier 13 (M13, NGC 6205), also called the
'Great globular cluster in Hercules', is one of the
most prominent and best known globulars of the Northern celestial hemisphere.

It was discovered by Edmond Halley in
1714, who noted that 'it shows itself to the naked eye when the sky is serene
and the Moon absent.'
According to Charles Messier, who
cataloged it on June 1, 1764, it
is also reported in John Bevis' "English"
Celestial Atlas.

At its distance of 25,100 light years, its angular diameter of 20' corresponds
to a linear 145 light years - visually, it is perhaps 13' large. It contains
several 100,000 stars; Timothy Ferris in his book Galaxies even says
"more than a million".
Towards its center, stars are about 500 times more concentrated than in
the solar neighborhood. The age of M13 has been determined by Sandage as
24 billion years and by Arp as 17 billion years around 1960; Arp later
(in 1962) revised his value to 14 billion years
(taken from Kenneth Glyn Jones).

According to Kenneth Glyn Jones, M13 is peculiar in containing one
young blue star, Barnard No. 29, of spectral type B2. The membership of
this star was confirmed by radial velocity measurement, and is strange
for such an old cluster - apparently it is a captured field star.

Observers note 4 apparently star-poor regions in M13 (e.g., Mallas).
Suggestions of them can be noted in some photos.

Globular cluster M13 was selected in 1974 as target for
one of the first radio messages
addressed to possible extra-terrestrial intelligent races, and sent
by the big radio telescope of the Arecibo Observatory.

Nearby, about 40 arc minutes north-east of M13, is the faint (mag 11)
galaxy NGC 6207, visible in many large- and medium-size-field photographs
of M13, e.g., in the
DSSM image.
This galaxy has recently produced a type II supernova (SN 2004A).